Electron tube employing a relatively long electron beam and getter material disposedat the collector



United States Patent 3,328,628 ELECTRON TUBE EMPLOYING A RELATIVELY LONG ELECTRON BEAM AND GETTER MA- TERIAL DISPOSED AT THE COLLECTOR Susumu Yasnda and Saburo Tsuchiya, Minato-ku, Tokyo, Japan, assignors to Nippon Electric Company, Limited, Minato-ku, Tokyo, Japan, a corporation of Japan Filed Oct. 5, 1962, Ser. No. 228,549 Claims priority, application Japan, Nov. 27, 1961, 36/ 43,012 3 Claims. (Cl. 315--3.5)

This invention relates to electron tubes, such as travelling-wave tubes, backward-wave tubes, multicavity klystrons and others which utilize a relatively long electron beam.

In conventional travelling-wave tubes (although applicable to the above described genus, the disclosure, for simplicity, will be directed to travelling tubes) it has been pointed out that even though the vacuum within the envelope is maintained as high as -10 mm. Hg, a bombardment of the cathode by a positive-ion beam, formed within the tube, gradually destroys the cathode surface and unduly shortens the life of the tube. It has been noted that the positive-ion beam also becomes a noise source in the travelling-wave tube during operation.

We have found that the positive-ion beam is produced in the following manner. Generally in the high vacuum of an ordinary discharge tube the very long mean free paths of the residual gas molecules reduces the probability of their collision with electrons travelling from the cathode to the anode. In tubes employing relatively long electron beams, however, t-he long beam enhances the probability of collisions between the electrons and gas molecules, resulting in the ionization of the gas molecules and the production of positive ions within the envelope. It is to be noted that the electron beam, focused by magnetic means, consists of a group of electrons taking helical paths, and having a certain diameter. The density of electrons is relatively higher at radially outward portions of the beam while it is relatively small at portions nearer to the central axis. Apparently the space charge of such electrons results in a potential distribution which shows a minimum at the center axis and a maximum along the radially outward circumference. Under such a potential distribution, the positive ions move towards the center axis of the electron beam and are caused to move towards the'cathode, or electron gun portion, where the potential is the lowest in the tube. This movement of positive ions produced all along the electron beam, results in an ion beam disposed along the center axis of the electron beam and flowing in beam form towards the cathode. Inasmuch as the positive ions not only have relatively large mass, but also large velocities near the cathode surface, their continued concentrated bombardment of the cathode surface produces an indentation. Where the cathode has an oxide layer the resulting hole has been found to penetrate this layer and erode the base metal. The positive-ion beam also applies a non-uniform component to the uniform flow of the electron beam and gives rise to a noise component in the high-frequency signal being amplified through the reception of the kinetic energy in the electron beam. It may thus be seen that the production of the positive-ion beam must be suppressed as far as possible.

Hence, it is the object of this invention to improve an electron tube in which a relatively long electron beam is employed so that destruction of the cathode and production of noise by the positive-ion beam is suppressed.

In the electron tu'be improved according to the invention, the construction is such that the positive ions produced therein flow not towards the cathode but towards "ice the collector or anode. Furthermore, the production of positive ions is reduced as far as possible by the provision, at the collector or anode, of a piece of metal which has excellent getter action and which, being sputtered by the positive-ion beam, continually absorbs the residual gas molecules.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the single figure which illustrates a travelling wave tube embodying the invention.

The travelling-wave tube shown in an axial section is similar in construction to one of conventional design except for its anode, which comprises a piece, later to be described, of active metal, such as titanium zirconium, magnesium, or hafnium, having excellent getter action with respect to the residual gas. More particularly, the travelling-wave tube comprises an electron gun 1; a helical electrode 2; a collector electrode 4, having at its exposed end portion a radiator 3; an envelope 5 enclosing such electrodes; and a magnetic focusing means 6 disposed in the stated order coaxially in the envelope 5. The high-frequency signal to be amplified is coupled to the helical electrode 2 through an input waveguide 7, while the amplified signal is available at output waveguide 8. The electrons emitted from the electron gun I move in an electron beam 9, composed of individual paths having different radii, toward the collector 4. The potential distribution in the cross section of the beam, as has been described, is relatively high along the radially outward circumference, relatively low at its center portion, and the lowest at the center. Inasmuch as the inside of the envelope 5 is evacuated to a high vacuum of 10 -10 mm. Hg, the mean free path of the residual gas is Very long. Nevertheless, the electrons have a relatively large probability of collision with, and consequent ionization of, the residual gas molecules due to their helical movements and their resultant elongated paths (it is to be noted that the collision probability has increased twice over ordinary tubes: by the relatively long gun to collector distance; and by the helical path of the individual electrons) from the gun to the collector. The positive ions produced by such ionization will crowd, under the influence of the above-mentioned potential distribution, the center axis of the electron beam 9. In-

asmuch as in a conventional travelling-wave tube the electric potentials of the cathode 11, the accelerating electrode 12, the helical electrode 2, and the collector 4 are in an ascending order, the positive ions will flow in a positive-ion beam 10 towards the cathode where the electric potential is the lowest in the travelling-wave tube. In the travelling-wave tube of the invention, however, the electric potential of the collector 4 is made considerably lower than that of the helical electrode 2 to prevent the positive-ion beam 10 from flowing towards the cathode 11. Since the lowering of the collector potential facilitates the production of secondary electrons (emitted from the surface of the collector by the electron beam bombardment) flowing towards the helical electrode in a direction opposite to the primary how of the primary electrons, it is necessary either to suddenly reduce the focusing magnetic field adjacent the collector or to make the magnetic field unsymmetrical. Using either of these methods of magnetic field discontinuity in proximity to the collector, it is possible to reduce the electric potential of the collector lower than that of the helical electrode so that the positive-ion beam flows mostly towards the anode or collector instead of the cathode.

The collector 4 is provided, on the surface facing the electron beam 9, with a recessed portion 13 for catching secondary electrons produced by the electron beam. The recessed collector method may be used either alone or together with either of the above-mentioned methods in order to solve the secondary collector emission problem. Disposed by either a forced-fit, cauling, or otherwise in the recessed portion is a piece of active metal 14, which shows excellent getter action with respect to the residual gas within the envelope. The positive-ion beam 10, upon reaching the anode, bombards the metal piece 14 and causes the active metal to sputter, with the result that the sputtered metal absorbs the residual gas. Continual sputtering of the active metal by the continuous flow of the positive-ion current maintains the getters evacuating action during the operation of the travelling-wave tube.

A test was conducted with a travelling-wave tube wherein the interior diameter and axial length of the helical electrode 2 were 2.4 mm. and 25 cm. respectively; the space between the helical electrode and collector 4 in the axial direction was 1 cm.; the axial dimension of the cylindrical recession 13 was 2 cm.; and the piece of active metal was a titanium disc of 5 mm. in diameter and 2 mm. in thickness retained in the recession by forced fit. The tube was operated with the focusing magnetic field abruptly reduced adjacent the collector 4, and with the electric potentials of the cathode 11, helical electrode 2, and collector 4 set at 0 v., 3,400 v., and 700 v., respectively. The collector current was about 20 ma., which showed that reduction of the collector voltage resulted in no harm. It was confirmed that the degree of vacuum rose, without a conventional barium getter (although one could, of course, be used in conjunction with the invention) from 6 1O mm. Hg, measured immediately after manufacture, to 1 10 mm. Hg (after several hours of aging); which showed that the aboveexplained getter action continued throughout the working time of the travelling-Wave tube.

As has been explained, a travelling-wave tube may have an increased life expectancy and low noise by lowering the collector voltage below the helical electrode voltage; by abruptly reducing the magnetic field for focusing the electron beam, by making such a field unsymmetrical in the area adjacent the collector or by providing the collector electrode with the recessed portion so that no secondary electron current flows from the collector to the helical electrode; and by providing the collector, on its surface facing the electron beam, with a piece of metal having excellent getter action.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. An improved electron tube of the type in which an electron beam emitted by a cathode electrode traverse a long beam path Within an evacuated envelope to a collector electrode, the collision of electrons of said beam with the residual gas in said envelope producing positive ions during the operation of said tube and wherein a high frequency circuit is electromagnetically coupled to said beam for the interchange of energy therewith, the improvement comprising: means for energizing said collector electrode with a potential which is lower than that of said high frequency circuit thereby to produce a potential gradient which will cause the positive ions to be attracted to said collector electrode and an active getter metal disposed on at least that surface of the collector electrode which is bombarded by the thus attracted positive ions.

2. The improvement claimed in claim 1 in which the collector is provided on the surface facing said beam with a recessed portion.

3. An electron tube comprising: an evacuated envelope; a cathode and a collector electrode positioned within said envelope; means for energizing said electrodes and for focusing the electrons emitted from said cathode into an electron beam; a high frequency circuit coupled to said electron beam for the interchange of energy therewith, the collision between said beam and residual gas molecules in said envelope producing positive ions; the collector electrode being supplied with a lower potential than said high frequency circuit thereby to produce a potential gradient in the vicinity of said collector electrode for attracting said positive ions; and an active getter metal positioned in said envelope for gettering the thus ionized gas molecules whereby damage to the cathode resulting from the impact of positive ions will be substantially reduced; the beam path between the cathode and collector electrodes being long and the getter metal being positioned in said evacuated envelope at the collector electrode.

References Cited UNITED STATES PATENTS 2,701,322 2/1955 Huber 313178 X 2,767,344 10/1956 Hines 315--3.5 2,813,990 11/1957 Robertson 3153.5 2,853,641 9/1958 Webber 315-3.5 2,991,391 7/1961 Beaver 315--3.5 2,996,639 8/1961 Jepsen 3l5-5.38 X

HERMAN KARL SAALBACH, Primary Examiner.

DAVID J. GALVIN, Examiner.

S. CHATMON, JR., Assistant Examiner. 

1. AN IMPROVED ELECTRON TUBE OF THE TYPE IN WHICH AN ELECTRON BEAM EMITTED BY A CATHODE ELECTRODE TRAVERSE A LONG BEAM PATH WITHIN AN EVACUATED ENVELOPE TO A COLLECTOR ELECTRODE, THE COLLISION OF ELECTRONS OF SAID BEAM WITH THE RESIDUAL GAS IN SAID ENVELOPE PRODUCING POSITIVE IONS DURING THE OPERATION OF SAID TUBE AND WHEREIN A HIGH FREQUENCY CIRCUIT IS ELECTROMAGNETICALLY COUPLED TO SAID BEAM FOR THE INTERCHANGE OF ENERGY THEREWITH, THE IMPROVEMENT COMPRISING: MEANS FOR ENERGIZING SAID COLLECTOR ELECTRODE WITHA POTENTIAL WHICH IS LOWER THAN THAT OF SAID HIGH FREQUENCY CIRCUIT THEREBY TO PRODUCE A POTENTIAL GRADIENT WHICH WILL CAUSE THE POSITIVE IONS TO BE ATTRACTED TO SAID COLLECTOR ELECTRODE 